On 15/11/2017 14:13, Russell King - ARM Linux wrote:

udelay() needs to offer a consistent interface so that drivers know
what to expect no matter what the implementation is.  Making one
implementation conform to your ideas while leaving the other
implementations with other expectations is a recipe for bugs.

If you really want to do this, fix the loops_per_jiffy implementation
as well so that the consistency is maintained.

Hello Russell,

It seems to me that, when using DFS, there's a serious issue with loop-based
delays. (IIRC, it was you who pointed this out a few years ago.)

If I'm reading arch/arm/kernel/smp.c correctly, loops_per_jiffy is scaled
when the frequency changes.

But arch/arm/lib/delay-loop.S starts by loading the current value of
loops_per_jiffy, computes the number of times to loop, and then loops.
If the frequency increases when the core is in __loop_delay, the
delay will be much shorter than requested.

Is this a correct assessment of the situation?

(BTW, does arch/arm/lib/delay-loop.S load the per_cpu loops_per_jiffy
or the system-wide variable?)

Should loop-based delays be disabled when CPUFREQ is enabled?

Regards.